Rebreathing circuits make it possible to reduce the amount of fresh gas delivered to a patient's lungs without raising blood carbon dioxide concentration. The reduction of fresh gas flow, in turn, conserves inhalational therapeutic agents. An example of this is the use of a rebreathing circuit to deliver volatile anesthesia. Other potential uses of rebreathing circuits include: efficient delivery of inhalational nitric oxide for pulmonary vasodilation, administration of helium gas as a means to reduce resistance to turbulent air flow in large airways, and reducing fresh gas flow as a means to limit evaporation of perfluorocarbon liquid from the lungs during partial liquid ventilation. Rebreathing circuits may also be used to enhance the delivery of aerosolized therapeutic agents for the delivery of drugs in particulate form to the lungs.
Rebreathing circuits are designed primarily to support anesthetic administration. In anesthesia applications, the gas in the rebreathing circuit is kept separate from the gas used to mechanically pressurize the respiratory circuit and thereby move the lungs. Were this separation to be incomplete or only partial, mixing of the gas streams might dilute the anesthetic being administered, which could result in the patient waking during surgery.
The prior art includes U.S. Pat. No. 4,989,597, which discloses a means to directly interface a ventilator to an anesthesia re-breathing circuit comprising a long, convoluted tube having a narrow diameter, yet large total volume. Such a device allows mixing of ventilator and re-breather gas streams. Under conditions of constant tidal volume, such a device causes a steady fractional admixture of the gas columns. Appropriate anesthetic concentration in the re-breather is maintained by delivery of an excess of anesthetic to the re-breather as compensation for losses due to mixing with the ventilator gas column. In that system, there is no divider between the separated gas columns. Instead there is an “open separation” of the gases resulting from the long mixing tube, which may contain two to three liters of gas.